AV Famicom strange expansion port controller issue

d0s

I have an AV Famicom (early version) that behaves strangely when a controller is plugged into the expansion port. There are phantom inputs, most commonly the right direction gets activated every few seconds but sometimes start or the A/B buttons will activate as well. I've tried it with three known good controllers (Hudson Joycard, ASCII Stick II Turbo, some Hori pad) with the same result, and this behavior doesn't happen when they're plugged into my Twin. This only happens when a controller is plugged into the expansion port, when there is nothing there or when using a controller plugged into the front NES style port it never happens. If I leave a controller pluggged into the player one front port and have a controller plugged into the expansion port the phantom inputs still happen but at a decreased rate, maybe every minute rather than every 5-10 seconds. Here is what I've tried to troubleshoot so far, all with no change:



- Desolder and reflow expansion port

- Desolder and reflow front ports

- Desolder and reflow all DA216 diodes

- Desolder and reflow anything that looks the slightest bit off

- Replace all electrolytic caps

- Check expansion port for visible damage and continuity

- Check continuity of traces coming from expansion port



I've tried google but have not found a single other person with the same problem and I'm kind of at the end of my rope, I'm very unfamiliar with the Fami/NES architecture. Any advice would be really appreciarted!

 

Duke.Togo

I've never heard of this issue before, but if you have tried all of that, the only other suggestion I would have is to probe the pins with an oscilloscope to try to find the fault. Good luck.

d0s

Just want to update this thread with the solution I found in case anyone else is having this issue. I tried a second AV Famicom and it exhibited the exact same behavior. However on both systems, plugging in the official AV Famicom dogbone controller into the player 1 front port completely fixes it (I was using a NES controller before, and it only reduced the amount of phantom inputs). I am guessing the Famicom hardware expects hardwired controllers due to how the original system had them, and it acts strange if you have a controller in the expansion port but nothing in the front port, and additionally that the Japanese dogbone controllers have some additional or different circutry that the original US NES controllers don't (I don't have a US dogbone to test). I would be interested to hear from other AV Famicom owners who own third party expansion port controllers like a Hudson Joycard to report back, as far as I know this has never been documented (at least, google turns up nothing)